Many people develop a lasting relationship with a piece of countryside, returning to it at different stages in their life, observing how it changes or stays the same. For Helen Young, the place is Farthing Downs on the southern fringes of London, a fragment of ancient chalk grassland famous for its skylarks, glow-worms and outsize snails.

She was no doubt taken there in her pram from her parents' home in Old Coulsdon, and clearly remembers going there on her first bike, which had stabilisers fitted on the rear wheels. She went there for picnics as a teenager and university student and still hasn't tired of the place.

"The nice thing is that it hasn't changed," she says. "The main reason I love it so much is that you'd never guess it was there. People think the whole area of Croydon and Purley is so built up and they can't imagine there's anything like this in the area. But when you're down in a bit called Happy Valley, you can't hear a thing from all the roads and you can really believe you're deep in the country.

"I go to get away from it all and hear the birds singing. My job is so demanding and it's good to remind yourself of nature and see a bit of open country. And here you can get into the country without having to go too far."

This kind of occasional escape has become more important now Young has taken over, at the age of 30, from Bill Giles as broadcast manager for the BBC Weather Centre, doing more high-profile TV work on BBC1 and organising the work of the rest of the team.

Her circuit of Farthing Downs starts at the northern end, a stone's throw from the main London to Brighton road. You walk south, rise quickly to 500ft, and pass Saxon burial mounds, an Iron Age field system and a clump of beeches called The Folly. "If you turn and look back back through the trees, you can see the London skyline," says Young.

She likes to follow the ridge for two miles to the tiny Chaldon church, which is mentioned in the Domesday book and contains a restored medieval mural showing Jacob's ladder, the long climb to Heaven. From there, returning north, you can drop to the right through fields into the secluded Happy Valley, with its horse tracks and clumps of hawthorn.

"Another reason why I like it there is that you can see the whole sky, which you can't do when you're in London," says Young. "There's a feeling of being very open to the elements and, of course, the first thing weather forecasters do when they go out is look at the sky.

One shadow over Farthing Downs, she says, is the official Met Office prediction that the next 50 years will bring hotter, drier summers, milder, wetter winters, and a greater chance of storms and gales. "Chalk grassland doesn't hold water too well and might have great problems with summer shortages - they might look very different for our children and grandchildren."

The M25 of walking

Farthing Downs is on the route of the London Loop, a new long-distance path that circles London with one foot in the outer suburbs and the other in the country. Waymarking and information leaflets have been completed south of the Thames and should soon be finished for the north as well.

The 150-mile route, designed by the London Walking Forum, is split into 24 sections that begin and end at a railway station or bus route, and more than 71% of early users said in questionnaires that this had prompted them to use public transport rather than cars. It is the pedestrian's answer to the M25.

On the southern section, from Erith, in the east, to Kingston, in the west, it crosses several fragments of chalk grassland that have survived the spread of suburbia and agriculture. Farthing Downs is one of a group of seven commons in Kent and Surrey bought for recreation by the Corporation of London in the 1880s.

Swathes of chalk grassland, which had been grazed for 5,000 years, have been lost to corn production since the second world war, and the decline of rabbits and grazing has eroded the variety of plant life: without grazing, there is a tendency for tor grass and stronger plants to dominate.

This is why the Corporation has brought sheep and cattle back on to Farthing Downs, and a consortium of local authorites, called the Downland Countryside Management Project, has done the same on other surviving fragments.

The main component of the downland turf is the springy fescue grass, which can give walkers the illusion that they're capable of walking all day.

 Information leaflets on sections 1-8 of the London Loop from Downlands Project, Highway House, 21 Chessington Road, West Ewell, Surry, KT17 1TT; tel: 020-8541 7282, enclosing SAE with 61p postage and a cheque for £6 made payable to Surrey County Council.

The practicals

Farthing Downs is close to Coulsdon South station: 30 minutes from London Charing Cross and 26minutes from London Victoria. For times of guided walks on the Down and a free information leaflet, contact Merlewood Estate Office, Ninehams Road, Caterham, Surrey CR3 5LN (020 8660 8533). Informatoin on local accomodation from Croydon Tourist Information Centre(020 8253 1000 or www.croydon.gov.uk